1. Field of the Invention
The invention relates to a highly effective method for removing and separating ammonia nitrogen from solutions.
2. Description of the Related Art
A number of industries produce ammonia waste, including ammonia plants in the petrochemical industry, coking plants in the steel industry, and fertilizer plants in the agrochemical industry. Ammonia is a dangerous water pollutant that adversely affects human health. Accordingly, much research has been focused on the development of methods for removal of ammonia from solutions.
These methods include microbial degradation, steam stripping, gas stripping, ion exchange, adsorption, electrolysis, membrane separation, precipitation, oxidation, breakpoint chlorination, wet oxidation, etc. Most of these methods work well in theory but in practice suffer from high operational cost and difficult implementation. Only the first three of the methods listed above, i.e., microbial degradation, steam stripping, and gas stripping, are used commercially, as illustrated below.
FlowConventionalConcentrationratetreatmentOperationalTypicalCategory(mg/L)(m3/h)methodEfficiencycostindustryHigh>100000-100SteamGoodHighAmmoniastrippingplantsMid-high200-100000-500Gas strippingLow; veryHighAmmonialow at lowplants;temperaturesCatalystfactoriesLow<2000-1000BiologicalGoodLowSewageand updegradationtreatmentplants
Ammonia nitrogen at high concentrations is amenable to steam stripping. Based on the difference in solubility at different temperatures, ammonia present in high concentrations is removed from the liquid phase by increasing solution temperature by means of steam. Meanwhile, ammonia is recycled or converted into ammonium salt generating revenue.
Ammonia nitrogen at low concentrations (mainly domestic sewage at 30-50 mg/L) is usually removed via biological methods. However, with the increase of NH3—N concentration, the operational cost increases significantly. For example, since 4.73 kg of O2 is theoretically required for removing 1 kg of ammonia nitrogen (while only 0.7-1.2 kg of O2 is required for removing 1 kg of BOD5), oxygen must be supplied by an air diffuser or an air fan, increasing energy consumption.
When NH3-N in waste water increases to about 70 mg/L, a carbon source concentration of 280 mg/L is required for removal of ammonia, while the BOD of regular domestic sewage is 150-250 mg/L. Therefore, extra carbon source (such as methanol) is required, which results in significant increase in operating costs. Hence, biological treatment works well only on NH3-N concentration of less than 100 mg/L. It should be pointed that when the NH3-N concentration is greater than 150 mg/L, the growth of common microbes is inhibited, which leads to poor removal of ammonia.
Mid-high concentration of ammonia often comes in waste water from waste leachate, and the petrochemical industry. There is no economical and effective conventional method of treatment for this type of waste water. If steam stripping is used, steam consumption is high relative to the value of recycled ammonia; if biological methods are employed, implementation is difficult and impracticable. Therefore, the gas stripping method is commonly used as the lesser of two evils.
The stripping method used to remove soluble ammonia from aqueous solutions is exemplified in FIG. 1. An external gas (carrier gas) is fed into a stripping tower where it passes against finely dispersed particles of ammonia-containing solution. In this way the gas-liquid interface is increased and soluble ammonia transfers from the liquid phase into the gas phase. Air is usually used as the carrier gas and the pH value of the ammonia-containing solution is adjusted to about 11 or higher by adding an alkali base, so as to convert ammonium ions (NH4+) dissolved in water to NH3 molecules. In the stripping tower, the aqueous solution is dispersed into small droplets or water mist, NH3 from the liquid phase is transferred into gas phase, and then carried away from the stripping tower with the carrier gas introduced by a blower.
The stripping tower is equipped with a packing layer having a certain height. Ammonia-containing solution is sprayed from the top of the tower, and flows downward along a surface of the packing. Air is blown from the bottom of the tower up, and continuously contacts with the solution. The disadvantages of the stripping method are include low efficiency (40-60% at normal temperature), and high operating cost due to the need to frequently replace the packing and clean the tower. Moreover, in winter when the temperature is low, the stripping method has very low removal efficiency due to a higher solubility of ammonia in colder water. Heating of waste water to remove ammonia in this process is not economical.
Since the gas-to-water ratio in the gas-stripping method is high, the energy consumption for this process is also relatively high. Normal cost for the gas stripping method of removal of ammonia nitrogen is about 1.5 USD or above per cubic meter. In addition, the concentration of residual ammonia nitrogen in waste water from which ammonia was removed by the gas stripping method is between 200-500 mg/L, and does not meet the discharge standard. Therefore, further treatment is often necessary.
GB Pat. Appl. Publ. No. GB2383034A describes a method for treating liquid containing ammonia, comprising the following steps: spraying an alkaline liquid from the center of a cylindrical vessel in the shape of an umbrella, allowing the water stream to hit the walls of the vessel, forcing air or nitrogen to flow in a tangential direction to the vessel walls, so as to form a cylindrical gas-liquid interface, allowing the gas to discharge from the top of the vessel, and the liquid to flow out downwardly from the bottom of the vessel. The GB application publication particularly emphasizes that the gas stream must enter in a tangential direction to form a spiral shape, and the vessel must be a cylinder without any obstacles therein.
However, the method does not use highly-dispersed micrometer- or nanometer-sized liquid particles. The nozzle of the stripping tower usually uses a reflective-I type, a reflective-II type or any other low water pressure nozzle with a water pressure of approximately 1 kg/cm2. To enlarge the gas-liquid contact surface, the stripping method relies on the generation of a liquid film (the liquid is liquid film or liquid drops with comparatively large particles instead of small particles), and the gas and the liquid form eddies in the cylinder to increase the contact surface. The efficiency is low and the operational costs are high.